The invention relates generally to the field of optical measurements and measuring systems, and more particularly to a system and method for optical heterodyne detection of an optical signal.
Dense wavelength division multiplexing (DWDM) requires optical spectrum analyzers (OSAs) that have higher spectral resolution than is typically available with current OSAs. For example, grating-based OSAs and autocorrelation-based OSAs encounter mechanical constraints, such as constraints on beam size and the scanning of optical path lengths, which limit the resolution that can be obtained.
As an alternative to grating-based and autocorrelation-based OSAs, optical heterodyne detection systems can be utilized to monitor DWDM systems. FIG. 1 is a depiction of a prior art optical heterodyne detection system. The optical heterodyne detection system includes an input signal 102, an input waveguide 104, a local oscillator signal 106, a local oscillator waveguide 108, an optical coupler 110, an output waveguide 118, a photodetector 112, and a signal processor 116. The principles of operation of optical heterodyne detection systems are well known in the field of optical heterodyne detection and involve monitoring the heterodyne term that is generated when an input signal is combined with a local oscillator signal. The heterodyne term coexists with other direct detection signals, such as intensity noise from the input signal and intensity noise from the local oscillator signal.
Optical heterodyne detection systems are not limited by the mechanical constraints that limit the grating based and autocorrelation based OSAs. The spectral resolution of an optical heterodyne system is limited by the linewidth of the local oscillator signal, which can be several orders of magnitude narrower than the resolution of other OSAs.
In order to improve the performance of optical heterodyne detection systems with regard to parameters such as sensitivity and dynamic range, it is best for the heterodyne signal to have a high signal-to-noise ratio. However, the signal-to-noise ratio of the heterodyne signal is often degraded by noise that is contributed from the direct detection signals, especially in the DWDM case where the input signal includes closely spaced carrier wavelengths. One technique for improving the signal-to-noise ratio of the heterodyne signal, as described in U.S. Pat. No. 4,856,899, involves amplifying the input signal before the input signal is combined with the local oscillator signal in order to increase the amplitude of the heterodyne signal. Although amplifying the input signal increases the amplitude of the heterodyne signal, the amplification also increases the intensity noise of the input signal and may not improve the signal-to-noise ratio of the heterodyne signal.
In view of the prior art limitations in optical heterodyne detection systems, what is needed is an optical heterodyne detection system that generates a heterodyne signal with an improved signal-to-noise ratio.
An optical heterodyne detection system includes an optical pre-selector that has an adjustable passband which is adjusted to track the wavelength of a swept local oscillator signal. In an embodiment, an input signal is combined with a local oscillator signal and the combined optical signal is filtered by the optical pre-selector. In another embodiment, the input signal is filtered by the optical pre-selector before the input signal and the swept local oscillator signal are combined. Filtering the input signal or the combined input signal and the swept local oscillator signal to pass a wavelength band that tracks the wavelength of the swept local oscillator signal reduces the noise contributed from wavelength division multiplexed (WDM) signals and increases the dynamic range of the optical heterodyne detection system.
An embodiment of the optical heterodyne detection system includes an optical combining unit, an optical pre-selector for the combined input and swept local oscillator signals, and a photodetector. In the embodiment, the input signal and the swept local oscillator signal are combined in the optical combining unit to create a combined optical signal. The optical combining unit includes one output for outputting a beam of the combined optical signal to the optical pre-selector. The optical pre-selector filters the beam of the combined optical signal and the passband of the optical pre-selector is adjusted to track the wavelength of the swept local oscillator signal. The photodetector is optically arranged to receive the filtered beam and generates electrical signals in response to the filtered beam. In an embodiment, an attenuator is utilized to attenuate the input signal.
Another embodiment of the optical heterodyne detection system includes an optical pre-selector for the input signal, an optical combining unit, and a photodetector. In the embodiment, the optical pre-selector filters the input signal while the passband of the optical pre-selector is adjusted to track the wavelength of the swept local oscillator signal. After the input signal is filtered, the filtered input signal and the swept local oscillator are combined in an optical combining unit. The optical combining unit outputs a beam of the combined optical signal to the photodetector and the photodetector generates electrical signals in response to the combined optical signal. In an embodiment, an attenuator is utilized to attenuate the input signal.
A method for monitoring an optical signal utilizing an optical heterodyne detection system involves combining the input signal and the swept local oscillator signal to create a combined optical signal and outputting the combined optical signal. An electrical signal is generated in response to the combined optical signal and the electrical signal is processed to determine an optical characteristic represented by the input signal. One of the combined optical signal, the input signal, and the swept local oscillator signal is filtered to pass a wavelength band that tracks the wavelength of the swept local oscillator signal. In an embodiment of the method, the combined optical signal is filtered and an electrical signal is generated in response to the filtered combined optical signal. In another embodiment of the method, the input signal is filtered and the filtered input signal is combined with the swept local oscillator signal to generate the combined optical signal. In an embodiment of the method, filtering of the optical signals is adjusted in real-time to track the wavelength change of the swept local oscillator signal. In an embodiment of the method, the input signal is attenuated before the input signal is combined with the swept local oscillator signal.
The optical heterodyne detection system and method provide an optical measurement system that is accurate over a wide range of wavelengths. The optical heterodyne detection system and method can be utilized for optical spectrum analysis to characterize an unknown input signal. The optical heterodyne detection system and method may also be utilized for optical network analysis in which a known signal is input into an optical network and the output signal is measured.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.